Your trunks are your lines. Be it SIP trunks, a PRI, a T1, CO/POTS, IP/XNET, they're all forms of trunks. Just like the phones, the trunks have a class of service (COS) to dictate what features should be enabled on them, and how they should behave.
What software version are you running? (This will help with telling you where to check on things.) You can find this info by clicking on the "About" link either in the lower-left corner or upper-right corner.